Powder injector

Abstract

A powder injector creates a powder-air mixture and feeds the mixture to a powder coating gun. The injector comprises a powder suction conduit, an injector nozzle fitted in the powder suction conduit, a mixing cone with a venturi tube connected thereto, the mixing cone being disposed downstream of the injector nozzle, a conveying air supply conduit including a control valve for feeding conveying air to the injector nozzle, and a metering air supply conduit for feeding metering air to metering air outlet ports opening into the venturi tube. The metering air supply conduit is composed of two parallel branch passageways branching off the conveying air supply conduit and opening into a common supply conduit leading to the metering air outlet ports. One of the two branch passageways has a metering air control valve disposed therein which is actuated by the air pressure prevailing in the conveying air supply conduit.

I claim as my invention: 
     
       1. A powder injector for forming a powder-air mixture and feeding said mixture to a powder coating gun, the powder injector comprising a powder suction conduit, an injector inserted in said powder suction conduit, a venturi chamber connected thereto, said venturi chamber being provided downstream of said injector, a conveying air supply conduit having a control valve therein for feeding conveying air at a pressure to said injector nozzle, and a metering air supply conduit for feeding metering air to metering air ports opening into said venturi chamber, comprising the improvement that: the metering air supply conduit is divided into two parallel metering air branch passageways, a first branch passageway and a second branch passageway, each branching off from the conveying air supply conduit at a location downstream of the conveying air control valve and opening into a common supply conduit to the metering air ports, and wherein flow-restricting orifice means for restricting air flow is inserted in one of said branch passageways, and wherein the second branch passageway has a metering air control valve disposed therein for controlling a flow cross section thereof, said metering air control valve being actuated by the pressure prevailing in the conveying air supply conduit downstream of said conveying air control valve.   
     
     
       2. The powder injector as claimed in claim 1, wherein the metering air control valve comprises a movable valve plunger for controlling the flow cross section in said second branch passageway, said plunger in one direction of movement thereof being biased by a spring and in another direction of movement thereof being influenced by the pressure prevailing in the conveying air supply conduit. 
     
     
       3. The powder injector as claimed in claim 2, wherein said orifice means is inserted in said first branch passageway and wherein an end face of said valve plunger of the metering air control valve projects into said first branch passageway upstream of the orifice means thereof. 
     
     
       4. The powder injector as claimed in claim 3, wherein a stroke of said valve plunger of the metering air control valve is adjustable by a regulating screw. 
     
     
       5. The powder injector as claimed in claim 3, wherein the spring of the metering air control valve exhibits a progressive characteristic. 
     
     
       6. A powder injector for forming a powder-air mixture and feeding said mixture to a powder coating gun, said powder injector comprising: a powder suction conduit;   an injector nozzle inserted in said powder suction conduit;   a mixing cone with a venturi tube connected thereto, said mixing cone being provided downstream of said injector nozzle;   a conveying air supply conduit having a control valve therein for feeding conveying air at a pressure to said injector nozzle; and   a metering air supply conduit for feeding metering air to metering air ports opening said metering air supply conduit into said venturi tube, the metering air supply conduit having a portion of a flow path defined thereby divided into two parallel metering air branch passageways, a first branch passageway and a second branch passageway, said portion being located in the flow path downstream of the conveying air control valve and flow connected to the metering air ports, and wherein one of the branch passageways has a metering air control valve disposed therein for controlling a flow cross section thereof of said one branch passageway, said metering air control valve being actuated by the pressure prevailing in the conveying air supply conduit downstream of said conveying air control valve.   
     
     
       7. The powder injector as claimed in claim 6, wherein the metering air control valve comprises a movable valve plunger for controlling the flow cross section in said one branch passageway, said plunger in one direction of movement thereof being biased by a spring and in another direction of movement thereof being influenced by the pressure prevailing in the conveying air supply conduit. 
     
     
       8. The powder injector as claimed in claim 7, wherein another of the branch passageways comprises an orifice means for restricting air flow disposed therein and wherein an end face of said valve plunger of the metering air control valve projects into said other branch passageway upstream of the orifice means thereof. 
     
     
       9. The powder injector as claimed in claim 8, wherein a stroke of said valve plunger of the metering air control valve is adjustable by a regulating screw. 
     
     
       10. The powder injector as claimed in claim 8, wherein the spring of the metering air control valve exhibits a progressive characteristic. 
     
     
       11. The powder injector as claimed in claim 6, wherein each of said branch passageways comprises an orifice means for restricting air flow disposed respectively therein. 
     
     
       12. A powder injector for forming a powder-air mixture and feeding said mixture to a powder coating gun, said powder injector comprising: a body member;   a powder suction conduit formed in said body member;   an injector nozzle inserted in said powder suction conduit;   a mixing cone with a venturi tube connected thereto, said mixing cone being provided downstream of said injector nozzle;   a conveying air supply conduit formed in said body and having a control valve therein for feeding conveying air at a pressure to said injector nozzle; and   a metering air supply conduit for feeding metering air to metering air ports opening into said venturi tube, the metering air supply conduit being divided into two parallel metering air branch passageways, a first branch passageway and a second branch passageway, each branching off from the conveying air supply conduit at a location downstream of the conveying air control valve and opening into a common supply conduit to the metering air ports, and wherein one of the branch passageways has a metering air control valve disposed therein for controlling a flow cross section of said one branch passageway, said metering air control valve being actuated by the pressure prevailing in the conveying air supply conduit downstream of said conveying air control valve.   
     
     
       13. The powder injector as claimed in claim 12, wherein said first branch passageway comprises an orifice restriction upstream of said common supply conduit, and said second branch passageway comprises a second orifice restriction upstream of said metering air control valve. 
     
     
       14. The powder injector as claimed in claim 12, wherein said conveying air supply conduit and said common supply conduit comprise parallel straight bores through said body and said first and second branch passageways are arranged bridging said parallel bores, and perpendicular to said parallel bores. 
     
     
       15. The powder injector as claimed in claim 14, wherein said conveying air supply conduit and said common supply conduit each has a check valve disposed therein. 
     
     
       16. The powder injector as claimed in claim 14 comprising a control valve bore arranged between said conveying air supply conduit and said common supply conduit and parallel thereto, said control valve bore intersecting said first and second branch passageways; and wherein the metering air control valve comprises a valve plunger slidably residing within said control valve bore and movable to block flow between said second branch passageway of the metering air supply conduit and said control valve bore, said metering air control valve comprising a spring biasing said plunger to a flow open position through said second branch passageway, said plunger having an end cap having an end face exposed to said first branch passageway and closing said first branch passageway from said control valve bore, said end cap being slidable upward by pressure in said first branch passageway, against a bias force of said spring, to position said plunger to close said second branch passageway, partially or completely.   
     
     
       17. The powder injector as claimed in claim 16, wherein said second branch passageway comprises a first segment between said conveying air supply conduit and said control valve bore, and a second segment between said control valve bore and said common supply conduit, said first segment and said second segment being offset in an axial direction of said control valve bore, and said control valve bore provides a valve outlet port in alignment with said second segment and said plunger provides an annular inlet groove alignable to said first segment. 
     
     
       18. A method for regulating conveying air and metering air in a powder injector for forming a powder-air mixture and feeding said mixture to a powder coating gun, the powder injector having a powder suction conduit having an air injector nozzle inserted therein, the injector nozzle receiving conveying air from a conveying air supply at a conveying air pressure to draw powder from a powder supply through the powder suction conduit, and a venturi tube downstream of said injector nozzle, the venturi tube having metering air ports for receiving metering air into the venturi tube interior from a metering air supply, comprising the steps: controlling a flow of the conveying air into the injector nozzle by regulating the conveying air pressure which is upstream of the injector nozzle, according to a powder demand;   controlling a flow of metering air into the metering air ports by supplying regulated conveying air as the metering air supply;   splitting the flow of metering air into two branches upstream of the metering air ports; and   controlling flow in one of the branches with a control valve closeable by increasing conveying air pressure.   
     
     
       19. The method according to claim 18, wherein the step of controlling flow in one branch is further characterized in that the control valve starts a closing movement when the regulated conveying air pressure is between 0.5 and 1.5 bar and is completely closed at a regulated conveying air pressure between 2.5 and 3.5 bar. 
     
     
       20. The method according to claim 18, wherein the step of splitting the flow of metering air into two branches is further characterized in that another of the branches is arranged for generally nonvariable flow resistance.